It is known that abbreviations and acronyms are frequently used in the mobile telephony field. A glossary of acronyms/terms which are used throughout the present specification is set forth below:    APN Access Point Name    CS Circuit Switching    GGSN Gateway GPRS Support Node    GPRS General Packet Radio Service    IMS IP Multimedia Subsystem    MO Mobile Originated    PDP Packet Data Protocol    PS Packet Switching    S-CSCF Serving Call Session Control Function    SGSN Serving GPRS Support Node    SIP Session Initiation Protocol    UMTS Universal Mobile Telecommunications System
Always-on is an old and well known problem in the PS environment. Theoretically, any user who executes a PDP Context Request (makes a request for a new context) and receives a positive response, is using IP resources in an always-on manner, i.e. always present and assigned, even if the user does not generate any IP data traffic. The type of resources used has more of a static character (IP address, memory load in the nodes, etc.) than a dynamic character (radio channels, etc.), however even so it is a question of scarce resources. In fact current GGSNs are configured to close any PDP Context without activity for the purpose of saving the maximum number of resources as possible. The element defining which inactive PDP must be closed is the APN with which the access is carried out.
When the IMS system is adopted on a large scale, it is possible that a paradoxical situation occurs. The use of the IMS for real time services implies an assumption of total reachability of the user at any time and in any place, with a view to maintaining the same level of service which the user currently has with CS. That means that the PDP Context for SIP signaling used by the user must always be available (from the IMS point of view, closing the signaling context means deregistering the user de facto), even if the inactivity of the user is extended for a long time. Therefore the IMS requirements will only worsen the problem of consuming IP resources without real associated traffic.
Before explaining the proposed solution to this problem by means of the present invention, what would occur with the current situation is described below. It is assumed that there is an IMS user already registered in the system and with active signaling context, therefore he or she is capable of initiating and receiving calls. If the GGSN decides to close that signaling PDP Context because the user has not carried out any additional interaction with the network, the user would pass to a status similar to that of deregistered, being unable to initiate or receive any call until requesting a new context and registering again in the system. This implies additional signaling and furthermore the expected delay upon setting up any type of call increases.